The invention relates to a semiconductor device for generating electromagnetic radiation in an active layer-shaped semiconductor region, in which the active region is composed of a layer structure having active layers of a first semiconductor material and of substantially equal thicknesses, which are located between and are separated by barrier layers of a second semiconductor material and of likewise substantially equal thicknesses having a larger energy gap than the first semiconductor material, both the active layers and the barrier layers comprising either a stoichiometric semiconductor compound or a semiconductor element and together constituting a superlattice structure.
A semiconductor device as described above is known from the international (PCT) Patent Application WO No. 82-03946 published on Nov. 11.sup.th, 1982.
Semiconductor devices for producing electromagnetic radiation are used in various fields. They are known in two types, i.e. devices whose emitted radiation is non-coherent and devices whose emitted radiation is coherent. The former devices are mostly designated as LED's (Light-Emitting Diodes), while the latter devices are designated as lasers. The wave-length of the emitted radiation can be situated in the visible part of the spectrum, but also, for example, in the infrared or ultraviolet part.
The pn-semiconductor lasers mostly used hitherto are so-called double hetero-junction (DH) lasers having a layer-shaped active region consisting of gallium arsenide or gallium aluminum arsenide situated between two cladding layers of gallium-alluminum arsenide of opposite conductivity type and having a larger energy gap (due to a higher aluminum content) than the active region. The gallium-aluminum arsenide used then has the composition A1.sub.x Ga.sub.1-x As, where x.ltoreq.0.45. In this interval for the atomic fraction x, the material is a so-called direct-gap semiconductor, which is a requirement for the occurrence of laser amplification.
The radiation produced by the aforementioned semi-conductor lasers has (in air) a wave-length of about 800 nm, in the infrared part of the spectrum. However, lasers and LED's producing radiation of a shorter wavelength, for example in the green, orange or visible red part, (620 nm) are in high demand. This is especially the case with the use of lasers for optically recording information on disks ("digital optical recording" or DOR), whereby holes are burned into a reflection layer by means of a laser beam. The attainable density of the stored information then increases in inverse proportion to the square of the wave-length of the radiation used. A decrease of the wave-length from, for example, 800 nm to 600 nm thus causes the density of the information that can be written to be approximately doubled.
In the aforementioned PCT Application No. 82,03946, a laser structure is described in which the layer-shaped active region consists of a comparatively large number of active layers of a stoichiometric semiconductor compound having a direct band gap, such as gallium arsenide, which are located between and are mutually separated by barrier layers of a stoichiometric semiconductor compound having an indirect band gap, such as aluminum arsenide. These active layers and barrier layers together constitute a so-called superlattice which has an effective energy gap lying between that of GaAs (1.35 eV) and that of AlAs (2.30 eV), i.e. 1.57 eV, so that the radiation produced has considerably shorter wave-length than would be the case with the use of an active layer consisting solely of gallium arsenide. This effect is attained by the occurrence of the so-called "quantum well" effect and of the "zone folding" effect in the active region.
The "quantum well" effect occurs when a very thin layer of a first semiconductor material is enclosed between two layers of a second semiconductor material having a larger energy gap than the first semiconductor material. It results in the effective energy gap in the very thin layer of the first material becoming larger and hence the wave-length of the radiation produced becoming smaller. The "zone folding" effect occurs due to the superlattice structure and results in the conversion of "indirect" semiconductor material into semiconductor material which is effectively "direct" with respect to the band transitions of charge carriers. This increases the radiating transition probability of the charge carriers so that a high radiation density can be attained. For a description of the "quantum well" effect, reference is invited inter alia to the article by Holonyak et al in IEEE Journal of Quantum Electronics. Vol. QE 16, 1980, p. 170-184.
For a description of the "zone folding" effect, reference is invited, for example, to the article by Osbourn et al in Applied Physics Letters, Vol. 41, (1982), p. 172-174.
The superlattice structure according to the aforementioned PCT Application WO No. 82-03946 consists of active GaAs layers having a thickness between 2 nm and 50 nm mutually separated by barrier layers of AlAs having a thickness between 1 nm and 20 nm. However, this combination of layer thicknesses is not suitable to obtain an optimum combination of "quantum well" effect and "zone folding" effect.